System and method for providing variable ultrasound array processing in a post-storage mode

ABSTRACT

An ultrasonic imaging method includes activating a transmit aperture within a multi-element transducer array, transmitting one or more ultrasonic beams along scan direction(s) that span the region of interest, for each transmit event, receiving ultrasound echoes from each element of a receive aperture, grouping the receive channel echo data into two or more sets corresponding to different receive sub-apertures, combining each sub-aperture data set to generate partially focused echo-location data for one or more reconstruction lines, and storing all the sub-aperture echo data sets during a storage period in a format that can be retrieved for later analysis. A method includes, during a post-storage period, retrieving stored sub-aperture data, combining the sub-aperture data to form one or more selected reconstruction lines, processing echo data to extract motion information from one or more sample positions along the selected reconstruction lines, and displaying an image representative of the processed motion information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Patent Application No.61/015,632 filed Dec. 20, 2008 for “System and Method for ProvidingVariable Ultrasound Array Processing in a Post-Storage Mode” (inventorsLarry Y. L. Mo, Glen W. McLaughlin, Brian Derek DeBusschere, Ting-LanJi, Albert Gee, David J. Napolitano, Ching-Hua Chou, and Robert W.Steins) the entire disclosure of which is herein incorporated byreference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to ultrasound imaging, and morespecifically to imaging techniques in a post-storage mode.

The following references are incorporated by reference:

-   U.S. Pat. No. 4,265,126: Papadofrangakis et al. (1981) “Measurement    of true blood velocity by an ultrasound system.”-   U.S. Pat. No. 4,604,697: Luthra et al. (1986) “Body imaging using    vectorial addition of acoustic reflection to achieve effect of    scanning beam continuously focused in range.”-   U.S. Pat. No. 5,365,929: Peterson (1994) “Multiple sample volume    spectral Doppler.”-   U.S. Pat. No. 5,409,010: Beach et al. (1995) “Vector Doppler medical    devices for velocity studies.”-   U.S. Pat. No. 5,415,173: Miwa et al. (1995) “Ultrasound diagnosis    system.”-   U.S. Pat. No. 5,690,111: Tsujino (1995) “Ultrasound diagnostic    apparatus.”-   U.S. Pat. No. 6,221,020: Lysyansky et al. (2001) “System and method    for providing variable ultrasound analyses in a post-storage mode.”-   U.S. Pat. No. 6,263,094: Rosich et al. (2001) “Acoustic data    acquisition/playback system and method.”-   U.S. Pat. No. 6,450,959: Mo et al. (2002) “Ultrasound B-mode and    Doppler flow imaging.”-   U.S. Pat. No. 6,679,847: Robinson et al. (2004) “Synthetically    focused ultrasonic diagnostic imaging system for tissue and flow    imaging.”-   U.S. Pat. No. 6,860,854: Robinson et al. (2005) “Synthetically    focused ultrasonic diagnostic imaging system for tissue and flow    imaging.”-   U.S. Pat. No. 6,926,671: Azuma et al. (2005) “Ultrasonic imaging    apparatus and method.”-   U.S. patent application Ser. No. 11/492,471, filed Jul. 24, 2006:    Napolitano et al. “Continuous Transmit Focusing Method And Apparatus    For Ultrasound Imaging System.”-   Tortoli et al. (1985) “Velocity profile reconstruction using    ultrafast spectral analysis of Doppler ultrasound.” IEEE    Transactions Sonics and Ultrasonics, vol. SU-32, pp. 555-561.-   Nitzpon et al (1995) “New pulsed wave Doppler u/s system to measure    blood velocities beyond the Nyquist limit.” IEEE Transactions    Ultrason., Ferroelec. and Freq. Cntrl., vol. UFFC-42, pp. 265-279.

In ultrasonic B-mode imaging, a two-dimensional (2D) image of tissue iscreated in which the echo intensity is mapped to pixel brightness. Forcontinuous wave (CW) measurement of blood flow in the heart and vessels,the frequency shifts of backscattered ultrasound waves are used toestimate blood velocities. For pulsed wave (PW) measurement of bloodflow, the phase shifts of backscattered ultrasound waves from a numberof transmit excitations are used for flow estimation. In 2D color flowimaging and timeline color-M mode, the mean phase shift, which isproportional to the motion-induced Doppler frequency shift, is displayedusing different colors that represent different flow speed. In spectralDoppler mode, the power spectrum of Doppler signals are computed anddisplayed as velocity-time waveforms. Contrast agents may be employedwith any of the imaging modes to further enhance the signal-to-noise orsignal-to-clutter ratio.

A typical ultrasound imaging system will include the following mainsubsystems: a transmitter, a receiver, a receive focusing unit, a cinememory buffer, an image processor/display controller, a display unit,and a master controller.

FIG. 1 is a block diagram of a conventional ultrasound system for whichthe receive array focusing unit is referred to as a beamformer, andimage formation is performed on a scan-line-by-scan-line basis. Systemcontrol is centered in the master controller, which accepts operatorinputs through an operator interface and in turn controls the varioussubsystems. For each scan line, the transmitter generates aradio-frequency (RF) excitation voltage pulse waveform and applies itwith appropriate timing across the transmit aperture (defined by asub-array of active elements) to generate a focused acoustic beam alongthe scan line. RF echoes received by the receive aperture of thetransducer are amplified and filtered by the receiver, and then fed intothe beamformer, whose function is to perform dynamic receive focusing;i.e., to re-align the RF signals that originate from the same locationsalong various scan lines.

To reduce the data sampling rate requirements, the RF data is oftenconverted (not shown) into baseband I/Q data before or after thebeamformer. In the ZONARE Z.one™ system, a synthetic array focusingapproach such as shown in U.S. patent application Ser. No. 11/492,471,filed Jul. 24, 2006 for “Continuous Transmit Focusing Method AndApparatus For Ultrasound Imaging System” (David J. Napolitano et al.) isused for 2D imaging. Specifically, a complete set of echo data obtainedfrom a sequence of transmit-receive cycles is accumulated in digitalmemory first, and then combined coherently in the receive focusing unitto produce the effect of having continuously focused transmit andreceive beams throughout the image field. Other methods of syntheticfocusing have also been described in U.S. Pat. Nos. 4,604,697,6,679,847, and 6,860,854, for both B-mode and flow imaging.

The image processor performs the processing specific to the activeimaging mode(s) including 2D scan conversion that transforms the imagedata from an acoustic line grid to an X-Y pixel image for display. ForSpectral Doppler mode, the image processor performs wall-filteringfollowed by spectral analysis of Doppler-shifted signal samples usingtypically a sliding FFT-window. It is also responsible for generatingthe stereo audio signal output corresponding to forward and reverse flowsignals. In cooperation with the master controller, the image processoralso formats images from two or more active imaging modes, includingdisplay annotation, graphics overlays and replay of cine loops andrecorded timeline data.

The cine buffer provides resident digital image storage for single imageor multiple image loop review, and acts as a buffer for transfer ofimages to digital archival devices. On most systems, the video images atthe end of the data processing path can be stored 20 to the cine memory.In state-of-the-art systems, amplitude-detected, beamformed data mayalso be stored 22 in cine memory. For spectral Doppler, some machinesstore the wall-filtered, baseband Doppler I/Q data for the user-selectedrange gate in cine memory, and the ZONARE z.one system stores thepre-wall-filtered data.

U.S. Pat. No. 6,263,094 describes a system wherein raw or partiallyprocessed data acquired early in the pipeline (pre-beamformed,post-beamformed, pre-video, etc.) is stored in non-volatile memory, andcan be introduced into a signal processing system from memory at leastat the rate that it was acquired, to produce a real time image that canbe modified by the reviewer by further processing, if desired. Thestrict requirement of introducing the acoustic data at least at thatparticular rate is to ensure the real time playback is “as if theinsonified target was the source of the acoustic data, not a storagedevice.” [4:30-31].

For motion imaging, the cost of storing Doppler data from earlier pointsin the receive data path is greater cine memory capacity (or shorterimage loops for the same memory size) and increased post-storageprocessing load (and response time). The benefits are encapsulated bythe concept of a “virtual patient;” i.e., the sonographer can focus onsetting the probe over the region of interest, and storing the raw(i.e., unprocessed) Doppler data first. Then, during cine review, theoperator can take time to re-adjust the image processing parametersbased on the same data set (the “virtual patient”) without keeping thepatient around longer than needed. Considering the fact thatsonographers need to manage many front-panel system controls with onehand, hold the probe with the other hand, and deal with the patient'sbody movements, the workflow and ergonomic advantages of suchpost-storage data processing capabilities are clear.

U.S. Pat. No. 6,221,020 describes a system wherein beamformed (I/Q orRF) data from a region of interest is accumulated 24 in cine memoryduring a storage period. As illustrated in FIG. 2, the receivebeamforming refers to a weighted delay-and-sum operation, and theamplitude weighting across receive aperture is referred to as“apodization,” which is important for suppressing the sidelobes of thereceive beam profile. This method enables, during post-storage playbackoperation 25, any known signal processing and system control which haveconventionally been carried out in real-time during the scanningsession. For example, in a playback mode, the user may select any scanline and Doppler gate location and width within the region of interestfor spectral Doppler or for color M-mode analysis. However, a limitationof this prior art is that the post-storage processing capabilities arerestricted to conventional methods based on ultrasound data that isalready beamformed.

Due to patient-to-patient variations and different anatomical scenariosin different clinical applications, a number of important systemcontrols actually need to be effected before or during the beamformingprocess. For example, it is well known that the sound speed parameterused for array focusing operations can vary with different organ typesand from patient to patient. Any sound speed optimization must beperformed during the receive array focusing or reconstruction process.In particular, the ZONARE z.one system supports channel domainprocessing that first stores all the raw transducer element data in achannel domain memory, and then allows digital signal processor units toaccess the channel data multiple times in order to support an iterativealgorithm that optimizes the array focusing sound speed. It is importantto note that this kind of adaptive processing may take up to a fewseconds to complete while the rest of the signal pipeline is paused;i.e., the channel data is often read out by the signal processorsmultiple times and at a rate that may be lower than when it wasacquired.

In spectral Doppler mode, the receive aperture, which determines thereceive F-number (defined by the ratio of focal range to aperture size),directly controls the degree of receive focusing. A lower F-number meansstronger focusing or a tighter beam width. For small vessels or weaksignals from deep lying vessels, it is advantageous to maximize thereceive aperture size for best sensitivity/penetration. On the otherhand, it is well known that because different elements of the activeaperture form different angles with the flow direction, a large aperture(low F-number) can give rise to an increased velocity over-estimationerrors—an effect referred to as geometrical spectral broadening.

U.S. Pat. No. 6,679,847 describes the synthetic focus system that storesraw channel data and applies different beamforming delay curves to trackblood motion. This invention is also aimed at providing flexible motionanalyses and display methods, but it is fundamentally a pure syntheticfocus system that utilizes single or small groups of elements fortransmit (with known sensitivity challenges associated with limitedacoustic power outputs) and requires an entire set of uncombined channeldata to be stored for each image frame.

SUMMARY OF THE INVENTION

There is a need for a more flexible system that can store data that isnot fully beamformed, and can introduce the stored data into a signalprocessing unit at a suitable rate that is not bound by the acquisitiondata rate, to provide variable Doppler array focusing adjustments andimage processing in a post-storage mode.

The present invention pertains to novel imaging capabilities in apost-storage mode, and is typically incorporated in an ultrasoundimaging system that includes the following main subsystems: atransmitter, a receiver, a receive focusing unit, a cine memory buffer,an image processor/display controller, a display unit and a mastercontroller.

Embodiments of the present invention provide a system and method foraccumulating partially reconstructed echo data from a region of interestduring a storage period. The selection of the region of interest may beeffected via standard user interface control mechanisms (e.g. trackballand “set” buttons), based on a real-time background B-mode and/or colorflow image display. The accumulated echo data representing the region ofinterest is then processed during a post-storage operation to provide anumber of user-selectable and/or adaptive array focusing strategies,motion analysis and display modes. The system further comprisesfront-panel controls that enable the user to select not only whichretrospective processing method to be used, but also one or more spatiallocations within the region of interest for such processing.

The techniques for accumulating partially reconstructed echo data from aregion of interest during the storage period (where results are storedfor subsequent processing in the post-storage phase) represent acommercially significant body of technology in their own right.Similarly, the techniques for retrieving and processing the partiallyreconstructed data during the post-storage period represent acommercially significant body of technology in their own right. Thus thetwo, which may be thought of as sub-combinations, should each be seen asa standalone aspect of the invention. While by definition accumulationand processing are done at different times, it is also contemplated thatthe two will often be done in different places by different entities.

In addition to the post-storage array focusing and image processingparameter adjustments, a number of advanced Doppler analysis and imagingmethods can benefit from this invention include Doppler spatialcompounding for improved velocity estimation, dual-frequency Doppler forextended velocity range, dual timeline Doppler, multi-gate spectral flowimaging, strain-rate imaging and adaptive post-processing includingadaptive clutter filtering and display parameters. These and othermethods have been reported in various published papers and are wellknown to those skilled in the art.

Hence, the potential benefits of this invention include not just all ofthe virtual patient capabilities, but also being able to analyze andprocess the same Doppler data using alternate processing and displaymethods aimed at extracting more diagnostic information.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional ultrasound system;

FIG. 2 is a block diagram showing prior art receive beamforming where aweighted delay-and-sum operation is performed;

FIG. 3 is a block diagram of a specific embodiment of the presentinvention;

FIG. 4 shows a preferred embodiment showing a motion imaging region ofinterest (“ROI”) for a curved scan format (left portion of figure) andfor a steered linear scan format (right portion of figure);

FIG. 5 shows a preferred embodiment in which the two receivesub-aperture channel sets are reconstructed and stored for each acousticgrid point;

FIG. 6 shows a preferred embodiment in which the receive channels aredivided into four sub-aperture groups for processing and storage;

FIG. 7 shows a preferred embodiment in which the region of interest(smaller box) for which partially beamformed PW Doppler data is storedis a sub-region of the standard color flow region of interest (largerbox), and also shows the time-line spectrograms for two Doppler samplevolumes along separate PW Doppler lines being displayed side-by-side;

FIG. 8 shows an anatomical reconstruction line within the ROI; and

FIG. 9 shows multiple sample volumes for quantitative Doppler waveformanalysis.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 3 is a block diagram of a preferred embodiment of the presentinvention for which system control is centered in a master controllerthat accepts operator inputs through an operator interface and in turncontrols the various major data processing subsystems. It is well knownto those skilled in the art that, through an user interface, theoperator can specify a region of interest (“ROI”) for a motion imagingmode (e.g., spectral Doppler, color velocity and energy imaging, andDoppler tissue imaging) within a background B-mode image. It is alsowell known that depending on the transducer type (curvilinear, linear orsector), a corresponding scan format is used to generate both the B-modeand motion imaging ROI.

FIG. 4 shows the examples of a curved and a linear array PW scan format,wherein the 2D motion imaging ROI defines a 2D acoustic grid for arrayfocusing or reconstruction operations. If the reconstruction is achievedusing a conventional beamforming method, the acoustic grid comprises aset of scan lines as illustrated for the case of the curvilinear scan.With ZONARE's imaging approach (see U.S. patent application Ser. No.11/492,471), the received data from all receive channels and transmitevents are first accumulated into a channel domain memory unit (notshown), such that the array focusing can follow an arbitrary acousticgrid as illustrated for the case of the linear scan (notice the lateraland/or axial spacing do not even need to be uniform). In general, thetransmit-receive array focusing and steering strategies including theacoustic reconstruction grid are independent of those that constitutethe background 2D image.

In general, this invention can be extended to 3D imaging such that theROI refers to a 3D volume in space. That is, the ROI is not restrictedto a 2D plane as shown in FIG. 4. For CW Doppler imaging, the ROI wouldtheoretically extend from the surface of the probe to infinite depth.

Transmitter and Receiver

The main functions of the transmitter and receiver in FIG. 3 are toacquire a set of echo data from the ROI over a sufficiently long timeinterval such that motion effects can be measured. For both CW and PWimaging modes, the ROI can be divided into one or more transmit scandirections, and it is well known that the overall lateral spatialresolution is determined by the product of the transmit and receivearray focusing response at each location.

In PW modes, a number of repeated transmit-receive events at auser-selectable pulse repetition frequency (PRF) is used to interrogatemoving sound reflectors along each transmit scan direction. That is, foreach spatial location within the ROI, a packet of echo data samples fromthe repeated transmit-receive events is available for analysis. Thenumber of data samples per packet is referred to as a “packet size.” Foreach packet, motion-induced echo changes from one sample to the next canbe detected. A motion estimate (e.g., mean velocity) can be derived foreach spatial location within the ROI, and the results can be translatedinto a 2D image, usually via a color mapping.

Suppose the ROI consists of 10 scan directions and suppose the packetsize is 12. This means that for each image frame, 10×12=120transmit-receive events are needed. In order to maximize the dataacquisition time, various well known acquisition strategies may be usedincluding multi-line and broad beam acquisition (one transmit broad beamspans two or more lateral positions in the acoustic reconstruction grid)and transmit interleaving (when the PRF is sufficient low such as fordeep imaging, a transmit pulse may be fired along a different scandirection before the echo from the previous firing returns). Typically,the RF echoes received by the receive aperture of the transducer areamplified and filtered by the receiver.

To enhance the axial resolution of the transmit pulse, and/or improvepenetration, coded excitation methods may be used wherein the transmitencoder (not shown) repeats a transmit pulse multiple times according toa signal modulation or coding scheme and a receive decoder (not shown)compresses the received signal thereby restores the uncoded pulseresolution but with greater signal to noise ratio. It is well known tothose skilled in the art that the receive decoding can be implementedbefore or after receive array focusing.

To reduce the data sampling rate requirements, the RF echo data is oftenconverted (not shown) into baseband I/Q data before or after receivebeamforming or array focusing. Optionally, as in the ZONARE z.onesystem, the received data from all receive channels and transmit eventscan be accumulated into a channel domain memory unit first prior tosub-aperture processing. Throughout this document, the term “imagereconstruction” will be used in relation to the preferred embodiments ofthe present invention, as the channel domain processing involved isdifferent and more general than conventional beamforming.

Sub-Aperture Processing

In contrast to the prior art of FIG. 2, an important aspect of thepresent invention lies with the sub-aperture processing of the receivedecho data from the different elements of the receive aperture. Asub-aperture refers to any subset (i.e., not necessarily contiguous) ofthe transducer elements that comprise the full receive aperture. Forexample, a sub-aperture may be defined by alternating elements acrossthe physical array, or by alternating groups of 2 or more elements. Thepartially focused sub-aperture data sets are then stored in the cinememory such that during a post-storage period, the user can process andre-process the stored data with new degrees of flexibility not availablewith conventional systems. “Subset” in this context means at least onetransducer element and fewer than all the transducer elements.

As illustrated by FIG. 5, one preferred embodiment is to divide thereceive aperture into two groups: an inner sub-aperture group (A) and anouter sub-aperture group (B). Each group is combined taking into accountof time delay, phase and amplitude weighting for optimal receivefocusing. During post-storage processing, the receive aperture caneffectively be adjusted by applying different weightings to the innerand outer sub-aperture. For flow imaging, this provides a means for theuser to adjust the tradeoff between flow sensitivity and geometricalspectral broadening error.

In another preferred embodiment as shown in FIG. 6, the receive channelsare grouped into a set of N contiguous sets of receive apertureelements. For example, suppose there are N=4 channels groups ofdifferent aperture sizes. For each acoustic grid point, 4 groups ofpartial reconstruction (apodized coherent sums) will be stored. A coarsesound speed tuning can be realized by adjusting the relative time delaysof the different sub-aperture groups during post-storage processing.That is, each sub-aperture group is regarded as a single receiver whosetime delay is given by twice the distance between the center of thesub-aperture to the desired focus location, divided by an assumed soundspeed value. This process can also be automated by implementing aniterative algorithm in the image processor that combines (withappropriate time delays) the sub-aperture data set multiple times usinga range of trial sound speed values, and automatically selects the bestsound speed that yields the best focusing in terms of a pre-determinedmetric (e.g., signal energy within the focus region).

As disclosed separately in U.S. patent application Ser. No. 11/492,471,reconstruction (sub-aperture reconstruction in the present case) is notlimited to receive array focusing, but may incorporate synthetictransmit array focusing, especially for slow flow conditions. That is,improvements in transmit array focusing can be achieved retrospectivelyby coherently combining channel data acquired from different transmitevents.

Data Compression and Storage

Referring to FIG. 3, after the sub-aperture processing, the partiallyfocused sub-aperture echo data can be optionally compressed via asuitable encoding method to reduce the total amount of data for cinememory storage. During cine playback or post-storage processing, thecompressed data is first decoded or un-compressed prior to furtherprocessing. Since ultrasound echo data is correlated across channels,across range, and across spatially adjacent transmit firings, and inaddition, in applications of color flow and spectral Doppler, the samereconstruction line is repeatedly sampled, there is a lot of redundantinformation present in raw echo data such that the opportunity forcompression is great. Many encoding/decoding techniques, both losslessand lossy, can be used including simple decimation, linear predictivecoding, discrete cosine transform, and wavelet transform. Some of thesemethods treat the data as a sequence of one's and zero's.

In a preferred embodiment of the present invention, the cine data isstored in non-volatile media on the system. In another preferredembodiment, the cine data, with or without compression, can betransferred to a remote workstation for post-processing and review. Thedata communications network connection to the remote review station canbe a wireless network.

Preferred Embodiments of Post-Storage Variable Array Processing andImaging Modes

One preferred embodiment of the present invention pertains topost-storage CW Doppler imaging capabilities. A single transmit broadbeam is used to define a relatively narrow ROI that may comprise two ormore reconstruction lines as defined by receive array focusing. Sinceonly one transmit beam is needed to scan the entire ROI, a CW transmitbeam can be used, and partially focused sub-aperture receive channelgroups of sampled I/Q data are stored in the cine memory. As an example,for a CW transmit frequency of fo=5 MHz, if the highest blood velocityto be measured is v=10 m/s (e.g., in cardiac scans), the correspondingmaximum Doppler frequency shift is 2(v/c)=64.9 kHz for c=1540 m/s (soundspeed in blood). Hence, to avoid aliasing the I/Q data may be sampled at100 kHz. During cine playback, the user can select via front-panelcontrol of the system a specific CW Doppler line within the ROI forspectral analysis. In effect, the CW Doppler line can be adjustedretrospectively after the raw data is acquired. Further, depending onwhich sub-aperture grouping method is used (FIG. 5 and FIG. 6), thisinvention also enables post-storage user-adjustments of receive aperture(geometrical spectral broadening errors) and receive aperture focusingsound speed.

In another preferred embodiment of the present invention, a post-storagePW Doppler mode wherein the sample gate position and/or receive aperturesize can be retrospectively adjusted upon cine playback. One or moretransmit beams are used to define a ROI within which a specific samplegate can be selected retrospectively for spectral analysis. If the PRFis sufficiently low, two or more PW transmit-receive cycles can beinterleaved; i.e., the PW transmit pulse for an adjacent beam can befired while waiting for the echo to return from the previous firing. Ifthe PRF is too high for interleaving, a single and broader transmit beammay be used such that it covers the entire ROI. The received channeldata for each transmit firing are grouped according to the inner andouter sub-aperture scheme of FIG. 5, with each group combined to formtwo or more reconstruction lines that span the ROI, and then stored incine memory. Upon cine playback, any sample gate along anyreconstruction line within the ROI can be selected via front-panelcontrols for spectral Doppler analysis as in live PW spectral Dopplerimaging. One exemplary application is, as shown in FIG. 7, is to enablecomparative and/or simultaneous blood flow analysis at two differentlocations relative to a stenosis in an artery. Two independent samplegates along two different reconstruction lines can be selected forspectral analysis, and the resultant time-line spectral waveforms can bedisplayed as a dual-image below the 2D color flow reference image.Regardless of the post-processing and display method, the receiveaperture size for a selected PW Doppler line can be adjusted by applyingdifferent weight factors to the partially focused inner and outersub-aperture echo data.

In another preferred embodiment, one or more transmit PW broad beams areused to define the ROI. The received channel data for all transmitevents are accumulated in a channel domain memory. With access to thechannel data memory, programmable processors (e.g., DSP chips, FPGA) areemployed to perform the sub-aperture reconstruction, which is no longerconstrained to follow a set of scan lines as in a conventionalhardware-based beamformer. As illustrated in FIG. 8, the reconstructionfollows an “anatomical line” that can be specified via front-paneluser-controls to be substantially normal to the near and far walls of avessel of interest, instead of a conventional scan line that is parallelto the transmit-receive beam. In general, an anatomical reconstructionline is chosen to follow the geometry of the anatomy of interest (e.g.,vessel cross-section) instead of a transmit-receive beam pattern. Duringcine playback, the user can select any arbitrary sample gate along theanatomical reconstruction line for detailed spectral analysis.Alternatively, a plurality of sample gates along a selected anatomicalline can be processed in parallel to produce a dynamic spectral flowprofile of range versus velocity, as taught by Tortoli et al. (1985),and also taught in U.S. Pat. No. 6,450,959.

As an extension to the preferred embodiments of FIG. 7 and FIG. 8, theuser can select two or more sample volumes in arbitrary positions withinthe ROI for quantitative spectral Doppler waveform index calculationsduring a post-storage period. Shown in FIG. 9 is an example in which abifurcating artery is being studied by placing a sample volume in thecommon vessel and one in a branch vessel. The time-line spectral Dopplerwaveforms corresponding to the two sample volumes can be optionallydisplayed as a dual image below the 2D image (as in FIG. 7). Anautomated maximum envelope or mean velocity waveform tracing algorithmis implemented to track various spectral waveform indices such asmaximum velocity (over a cardiac cycle) and the systolic to diastolicratio. In FIG. 9, the maximum velocities for the two sample volumes aredisplayed below the 2D image.

Another preferred embodiment of the present invention is retrospective2D color flow or tissue motion imaging. The acquisition strategies andoptions are similar to the aforementioned PW Doppler modes, except thedisplay is in the form of a colorized flow image or a 2D grid of samplegates within the ROI. A packet of say, 10 transmit-receive events isacquired for each motion image frame, and another packet for the nextframe etc. For each image frame, a 2D grid of sample gates is analyzedserially or in parallel to estimate motion parameters such as meanvelocity and motion signal energy. The processing may include amotion-discrimination filtering step including a high-pass “clutter”filter for flow imaging, and a low-pass “flow-signal” filter for tissuemotion imaging. The results are displayed using a pre-determined colormap to provide a visual representation of the spatial distribution ofthe flow parameters within the ROI.

Another preferred embodiment of the present invention involves use oftwo or more independent sets of reconstruction lines that intersect eachother within the ROI. It is well known to those skilled in the art thatsub-aperture data obtained from two or more independent views of asample volume can be combined to estimate the velocity vector (magnitudeand direction) and/or to reduce estimation variance. By storing thesub-aperture data sets corresponding to different reconstruction linesin cine memory, the improved velocity estimation can be effected in apost-storage mode.

Other advanced Doppler analysis and imaging methods that can benefitfrom the present invention include dual-frequency Doppler for extendedunambiguous velocity measurement range, and adaptive post-processingincluding adaptive receive aperture, range-gating, PRF re-sampling, anddisplay parameters. For example, using the inner and outer sub-aperturemethod of FIG. 5, an adaptive receive aperture can be realized byprescribing weight factors for the two sub-aperture groups based on anestimate of the motion signal parameter (e.g., signal energy, mean ormaximum Doppler frequency). If the flow signal is strong, the weightingon the outer sub-aperture group may be reduced relative to the innersub-aperture group thereby reduce geometrical spectral broadeningerrors.

In the foregoing specification the invention has been described withreference to specific exemplary embodiments. It will, however, beevident that various modifications and changes can be made withoutdeparting from the broader spirit and scope of the invention as setforth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense.

The invention claimed is:
 1. A method of analyzing movement of blood oranatomical structures or both within a region of interest, operating inconjunction with B-mode or color flow imaging modes or both, the methodcomprising: activating a transmit aperture within a multi-elementtransducer array; transmitting one or more ultrasonic beams along one ormore scan directions that span the region of interest; for each transmitevent, receiving ultrasound echoes from each element of a receiveaperture; grouping the receive channel echo data into two or more setscorresponding to a first set of receive sub-apertures; combining eachsub-aperture data set to generate partially focused echo-location datafor one or more reconstruction lines; storing all the sub-aperture echodata sets during a storage period in a format that can be retrieved forlater analysis, such that the stored, partially focused echo data setsare capable of being recombined according to a second, different set ofreceive sub-apertures; and determining a sound speed value byiteratively focusing data of two or more receive sub-apertures underdifferent respective trial sound speed values.
 2. The method of claim 1,and further comprising: during a post-storage period, retrieving storedsub-aperture data at a rate that is not bound by the rate at which thesub-aperture data was acquired; and combining the sub-aperture data toform one or more selected reconstruction lines; processing echo data toextract motion information from one or more sample positions along theselected reconstruction lines; and displaying an image representative ofthe processed motion information.
 3. The method of claim 2, and furthercomprising: selecting, in response to user signals, one or more spatiallocations within the region of interest for retrospective processingduring the post-storage period; adjusting, in response to user signals,the post-storage receive array focusing and motion-analysis modes andassociated processing parameters.
 4. The method of claim 1 whereintransmitting one or more ultrasonic beams comprises generating asequence of ultrasound pulses designed to adequately sample a movingtarget with a given maximum velocity in the direction of the transmitbeam.
 5. The method of claim 1 wherein transmitting one or moreultrasonic beams comprises generating a continuous acoustic wave of apre-determined transmit frequency.
 6. The method of claim 1 whereintransmitting one or more ultrasonic beams comprises generating a weaklyfocused or broad beam that spans two or more reconstruction lines. 7.The method of claim 1 wherein transmitting one or more ultrasonic beamscomprises generating two or more ultrasound beams transmitting the beamssuch that they cross over each other within the region of interest. 8.The method of claim 1, and further comprising the step, performed priorto storing given sub-aperture echo data, of converting the received echodata from RF to baseband I/Q components.
 9. The method of claim 1,further comprising accumulating and holding the received ultrasound echodata from each transducer element from one or more transmit events in achannel domain memory until said partial focusing operations for anentire image frame are completed.
 10. The method of claim 1 whereincombining sub-aperture data set to generate partially focusedecho-location data includes combining channel data obtained from two ormore transmit events.
 11. The method of claim 1, and further comprisingthe step, performed prior to storing a given sub-aperture echo data set,of compressing the given sub-aperture echo data set.
 12. The method ofclaim 1 wherein grouping the receive channel echo data into two or moresets comprises dividing the receive aperture into two channel groups: afirst group that represents an inner sub-aperture of the receive array,and a second group that comprises two outer sub-apertures.
 13. Themethod of claim 1 wherein grouping the receive channel echo data intotwo or more sets comprises dividing the receive aperture into Ncontiguous groups of elements, where N is greater than or equal to two.14. The method of claim 1 wherein combining sub-aperture data set togenerate partially focused echo-location data for one or morereconstruction lines comprises constructing at least one reconstructionline that does not follow the direction of a transmit beam.
 15. Themethod of claim 1, and further comprising the step, performed prior tostoring a given sub-aperture group, of combining the given sub-aperturegroup with the appropriate time delays, phase shifts and amplitudeweightings to form partially focused lines along one or morereconstruction lines.
 16. The method of claim 1 wherein storing thepartially focused sub-aperture echo data comprises storing the partiallyfocused sub-aperture echo data in non-volatile media on the system or ona remote review station that is connected to the system via a digitalcommunications network.
 17. The method of claim 1, and furthercomprising, during a post-storage period, combining the sub-aperturedata for the selected reconstruction line(s) with different amplitudeweightings to control the overall receive aperture apodization functionof the resultant reconstruction line(s).
 18. The method of claim 1,wherein determining the sound speed value comprises selecting one of aplurality of trial sound speeds based on focus metrics of the two ormore receive sub apertures under the respective trial sound speeds. 19.The method of claim 2, and further comprising prior to the sub-aperturedata combining step, of performing some processing of the echo data foreach sub-aperture.
 20. The method of claim 1, and further comprisingprocessing multiple sample gates along the same reconstruction linesimultaneously to produce a spectral flow profile.
 21. The method ofclaim 1, and further comprising processing different sample gates withinthe ROI in parallel.
 22. The method of claim 21, and further comprisingdisplaying time-line spectral waveforms corresponding to the differentsample gates using multiple image display windows.
 23. The method ofclaim 21, and further comprising: automatically computing Dopplerspectral waveform indices over one or more cardiac cycles; anddisplaying the spectral waveform indices for two or more sample gateswithin the ROI.
 24. The method of claim 1, and further comprisingprocessing the corresponding sample gates defined by the region ofintersection between two or more transmit-receive cross reconstructionlines such that echo data from the different views are combined toimprove motion estimation including variance reduction and flowdirection or Doppler angle estimation.
 25. A system for analyzingmovement of blood or anatomical structures or both within a region ofinterest, operating in conjunction with B-mode or color flow imagingmodes or both, the system comprising: a processing device; and a storagedevice; said processing device operating, during a storage period, to:generate signals that result in activating a transmit aperture within amulti-element transducer array; generate signals that result ininstructing the transducer array to transmit one or more ultrasonicbeams along one or more scan directions that span the region ofinterest; for each transmit event, receiving data representingultrasound echoes from each element of a receive aperture; grouping thereceive channel echo data into two or more sets corresponding to a firstgroup of different receive sub-apertures; combining each sub-aperturedata set to generate partially focused echolocation data for one or morereconstruction lines; storing all the sub-aperture echo data sets insaid storage device in a format that can be retrieved for lateranalysis, such that the stored, partially focused echolocation data isconfigured to be recombined into a second group of different receivesub-apertures; and determining a sound speed value by iterativelyfocusing data of two or more receive sub-apertures under differentrespective trial sound speeds.
 26. A method of analyzing movement ofblood or anatomical structures or both within a region of interest, themethod comprising: retrieving sub-aperture data generated during aprevious storage period where the sub-aperture data represents partiallyfocused echo-location data resulting from grouping receive channel echodata from elements of a receive aperture into different receivesub-apertures that follow a first set of scan lines; determining a soundspeed by selecting one of a plurality of trial sound speed valuesresulting in an optimal focus of one or more of the sub-apertures;combining the sub-aperture data to form one or more selectedreconstruction lines, wherein the selected reconstruction linescorrespond to a second set of scan lines that differ from the first setof scan lines; processing echo data to extract motion information fromone or more sample positions along the selected reconstruction lines;and displaying an image representative of the processed motioninformation.
 27. The method of claim 26 wherein the sub-aperture datahad been stored in a compressed format, and further comprisinguncompressing the sub-aperture data prior to further processing duringthe post-storage period.
 28. The method of claim 26, and furthercomprising after retrieving sub-aperture data, combining thesub-aperture data, combining the sub-aperture data from the selectedreconstruction line(s) within different amplitude weightings to controlthe overall receive aperture apodization function of the resultantreconstruction line(s).
 29. The method of claim 26, and furthercomprising after retrieving sub-aperture data, combining thesub-aperture data for the selected reconstruction line(s) with differenttime delays and phase shifts based on an assumed sound speed value. 30.A system for analyzing movement of blood or anatomical structures orboth within a region of interest, operating in conjunction with B-modeor color flow imaging modes or both, the system comprising: a processingdevice; and a storage device; said processing element operating, duringa post-storage period, to: access a storage device storing sub-aperturedata generated during a previous storage period where the sub-aperturedata represents partially focused echo location data resulting fromgrouping receive channel echo data from elements of a receive apertureinto different receive sub-apertures that correspond to a first set ofscan lines; retrieve from said storage device stored sub-aperture dataat a rate that is not bound by a rate at which the stored sub-aperturedata was acquired; determine a sound speed by selecting one of aplurality of trial sound speed values resulting in an optimal focus ofone or more of the sub-apertures; combine the sub-aperture data to formone or more selected reconstruction lines that correspond to a secondset of scan lines different from the first set of scan lines; processecho data to extract motion information from one or more samplepositions along the selected reconstruction lines; and generate signalsthat result in displaying an image representative of the processedmotion information.
 31. A method of analyzing movement of blood oranatomical structures or both within a region of interest, operating inconjunction with B-mode or color flow imaging modes or both, the methodcomprising: activating a transmit aperture within a multi-elementtransducer array; transmitting one or more ultrasonic beams along one ormore scan directions that span the region of interest; for each transmitevent, receiving ultrasound echoes from each element of a receiveaperture; grouping the receive channel echo data into two or more setscorresponding to a first set of different receive sub-apertures;combining each sub-aperture data set to generate partially focusedecho-location data for one or more reconstruction lines; storing all thesub-aperture echo data sets during a storage period, such that thestored, partially focused echo location data sets are capable of beingrecombined according to one or more different sets of different receivesub-apertures; combining the sub-aperture data to form a second set ofreceive sub-apertures corresponding to one or more selectedreconstruction lines during a post-storage period; determining a soundspeed value corresponding to the stored sub-aperture echo data sets byiteratively focusing the first set of sub-apertures under differenttrial speed values and selecting the trial speed value that yields anoptimal focusing result; processing echo data to extract motioninformation from one or more sample positions along the selectedreconstruction lines; and displaying an image representative of theprocessed motion information.